Implementation of HONO Chemistry into a Chemistry-climate Model CHASER and Its Impacts on Tropospheric Chemistry

Abstract:

Nitrous acid (HONO) is an essential trace gas in the troposphere, which is produced not only through gas-phase reactions but also from heterogeneous processes. The importance of HONO chemistry in ozone modelling has been suggested by many previous studies. However, many of current atmospheric models still fail to fully simulate the HONO chemistry, especially for its heterogeneous processes.

This study newly implements the simulation of HONO into a chemistry-climate model CHASER (MIROC-ESM) (Sudo et al., 2011). In the model, the HONO formation is driven by the gaseous reaction of NO+OH and heterogeneous reactions of NO₂ on the surfaces of cloud, sulphate, dust, organic and soot particles. The model also includes chemical loss processes of HONO through photolysis, the gaseous reaction with OH and its heterogeneous reaction on cloud droplets, ice crystals and sulphate particles (HONO → NO). The terrestrial emissions of HONO (~0.7 TgN/yr) are tentatively given based on an inventory of soil NO_x emissions.

The newly added HONO system significantly reduced the model bias in the lower troposphere (>600 hPa) against the aircraft measurements of NASA’s ATOM-1 for the regions of Pacific, Atlantic and North America (8/2016) for NO₂ (22%), OH (72%), O₃ (59%), and CO (57%). The newly implemented HONO chemistry also reduced the model bias against EANET and EMEP stationery observations in East Asia and Europe regions for O₃ concentrations (41% and 13% respectively).

This study suggests that the whole HONO system (incl. heterogeneous reactions) increases the global mean CH₄ lifetime by 15.1%, and changes the global abundances in NO_x, O₃ and CO by -17.2%, -6.2% and +11.8% respectively. In JJA (June, July, August), significant changes for OH, NO_x, O₃, and CO by up to -68%, -94%, -43%, and +19% respectively, are seen on the surface of North Pacific region due to the impacts of HONO chemistry. In DJF (December, January, February), HONO chemistry also reduced O₃ level on the surface of the Chinese area by up to -75%. The separate impacts of emission, gaseous reactions and heterogeneous reactions of HONO will also be discussed in the presentation.